Move and rename parse_path_argument() function

This fits better in shared/, and the new parse-argument.c file is a good home
for it.

src/systemctl/systemctl.c

@@ -638,7 +638,7 @@ static int systemctl_parse_argv(int argc, char *argv[]) {
                         break;
 
                 case ARG_ROOT:
-                        r = parse_path_argument_and_warn(optarg, false, &arg_root);
+                        r = parse_path_argument(optarg, false, &arg_root);
                         if (r < 0)
                                 return r;
                         break;